The present invention relates to the art of converting lignocellulosic materials, such as wood, into pulp, more specifically to those processes employing at least some chemicals to convert the lignocellulosic materials into the desired pulp.
The commercially valuable processes for the chemical pulping of lignocellulosic materials, commonly wood chips, are normally referred to as the kraft process, the soda process and the sulfite process. There are also pulping processes which employ a combination of chemical and mechanical pulping steps and these processes are sometimes referred to as semi-chemical or chemi-mechanical pulping processes. These processes use some of the same chemicals as the kraft, soda and sulfite processes.
For a number of reasons, the preferred chemical pulping process is the kraft process which involves cooking or pulping appropriately comminuted pieces of lignocellulosic material, e.g. wood chips, in an aqueous alkaline solution of sodium hydroxide, sodium carbonate, and sodium sulfide. Normally the process is carried out in a pressure vessel called a digester in which the contents are heated to temperatures of about 160.degree. to 180.degree. C., for about one to three hours. Following the cooking or pulping stage the cooking liquor is separated from and to a greater or lesser extent washed out of the pulp and is then subjected to a recovery treatment to recover the chemical and energy values. Because of its dark color the pulping liquor is known as kraft black liquor.
The sulfite process comprises cooking or pulping appropriately comminuted lignocellulosic material in an acidic aqueous solution of sulfur dioxide together with chemicals providing calcium, magnesium, sodium, or ammonium ions. The aqueous solution, thus, contains sulfurous acid, sulfite and bisulfite ions. The cooking period requires from about six to about eight hours during which time the temperature rises to about 140.degree. C. In a variation of this process the cooking liquor may be made neutral or mildly alkaline. Recovery of the spent pulping liquors has been accomplished by a number of techniques. In general, recovery of chemical values from spent liquors of the sulfite process has proven more difficult than recovery of chemical values from the black liquors of the kraft process. This is a significant reason for the predominance of the kraft process over the sulfite process for pulping lignocellulosic material.
The traditional recovery process for kraft black liquor has employed the so-called Tomlinson kraft recovery boiler. In this boiler, concentrated black liquor serves as fuel to provide heat for general process use. The combustion process produces, in addition to the heat generated from the combustion of organic matter present, a smelt or molten body of inorganic chemical which comprises sodium carbonate and sodium sulfide. The furnace process essentially consists of two stages, an initial combustion stage wherein the inorganic salt residue comprises sodium carbonate and sodium sulfate and a second reduction stage wherein sulfate is reduced to sulfide. The molten smelt from the reduction stage is dissolved in water to produce so-called green liquor which is then treated with lime to convert some sodium carbonate to sodium hydroxide thus converting the solution into white liquor after separation of calcium carbonate. The white liquor may then be employed as a kraft cooking or pulping liquor useable in future pulping operations after replenishment of any depleted components.
The Tomlinson recovery furnace is less than an ideal solution to the problem of kraft black liquor energy and chemical recovery for several reasons. These are briefly, an opportunity exists for serious explosions if water inadvertently contacts molten inorganic salts, recovery of energy values is reduced, emission of reduced sulfur compounds to the atmosphere requires extensive odor control and independent control of the physical and chemical actions present in the process is not possible since both oxidation and reduction steps are being carried out in a single vessel within close proximity.
Despite the capital investment in existing recovery systems, it is therefore not surprising that the paper industry has and is investigating alternative recovery possibilities. One such alternative is a multiple solids fluidized bed recovery system described in U.S. Pat. No. 4,303,469 and in U.S. patent application Ser. No. 342,954 filed Jan. 26, 1982. The present invention provides an alternative technique for the operation of the apparatus and of the processes described and claimed therein. In the recovery system of the patent and the application, concentrated spent pulping liquor is combusted in a fluidized bed reactor, employing multiple inert solid components, one of which may be referred to as being of fine particles and the other of coarse particles. The spent liquor is introduced at the bottom of an initial fluidized bed reactor which also contains the plurality of inert solid particle components and is subjected to the introduction of sufficient air to support a substantial combustion of most, but not all, of the organic matter contained in the concentrated spent pulping liquor. Desirably, only about 80 to 90 percent, based on carbon content of the organic material is combusted in the initial fluidized bed. The plurality of inert particulate solid components, more particularly, the finer fraction of said particulate solids, as well as the combustion gases and uncombusted material from the initial fluidized bed reactor, are removed from the top of the fluidized bed reactor and subjected to a separation of most of the inert solids from the mixture of gases and, if necessary, from uncombusted solids from the combusted spent liquor.
When providing for incomplete combustion in the initial fluidized bed reactor, as described above, so as to provide for the formation of uncombusted carbonaceous material, the uncombusted carbonaceous material is intended to provide a reductant for the conversion of sulfate, formed in the initial combustion of the spent liquor, to sulfide.
The separated inert solids are then transmitted to one or more fluidized beds acting as external boilers. This unit or units may preferably contain immersed heat exchange tubes and steam may be generated from the sensible heat released by the solids. The external boiler or boilers remove a significant portion of the residual heat value produced in combustion in the initial fluidized bed reactor and contained in the finer inert solids. These finer inert solids, with much of their heat value recovered, are returned to the bottom of the initial fluidized bed reactor where they are contacted with the air and concentrated spent liquor and refluidized.
The employment of the plural-stage fluidized bed reactors, the initial stage of which employs two sizes of solid particulate components, performs the role of absorbing heat of combustion and as distinguished from conventional fluidized bed reactors some of the solids, as well as the gaseous components are removed at the top of the reactor, instead of the bottom. In this way, combustion is carried out efficiently at high gas velocity without the need of internal heat removal surfaces in the initial fluidized bed reactor. Conventional fluidized bed reactors remove heat via tubes embedded in the reactor which have short useful lives requiring frequent replacement with attendant downtime and these, depending on the operating conditions, may also hinder the fluidization process. The plurality of solid particulate components employed in the initial fluidized bed reactor are inert and perform the roles primarily of recovering heat from the combustion process and providing excellent mixing of air and concentrated spent pulping liquor.
A suitable multiple solid fluidized bed reactor for use in the present invention is disclosed in Nack, et al., U.S. Pat. No. 4,084,545. Suitable multiple inert solid components are disclosed in the aforementioned U.S. Pat. No. 4,303,469 as are typical operating conditions.
The present invention provides an alternative apparatus configuration and process for treating incompletely combusted carbonaceous and sulfurous materials in the flue gases resulting from the processes of the recovery system and for recovery of very fine residual particulate matter entrained in the flue gases.